The present invention relates to a method and a circuit arrangement for determining a direction of an exterior magnetic field.
The use of magnetoresistive elements in the context of angle sensors is known. For instance, on the basis of the AMR (anisotropic magnetoresistance) effect, angle sensors furnish output signals from which the direction of an externally applied magnetic field can be determined. If the angle of the magnetic field relative to the sensor is xcex1, then the sensor produces output signals that are proportional to sin(2xcex1) and cos(2xxcex1). AMR sensors that function in this way furnish a nonambiguity range of 180xc2x0 available, based on the periodicity of the sine and cosine signals. In a further development of such AMR sensors, as disclosed for instance in German Patent Application DE 198 39 446, which had not yet been published by the priority date of the present application, these sensors are embodied with integrated auxiliary coils.
By application of a suitable current to the auxiliary coils, individual, slight additional magnetic fields are generated in the various sensor halves; these fields are superimposed on the exterior magnetic field to be measured and change the direction of the resultant field. By observation of the change in direction, the nonambiguity range of the sensor can be increased from 180xc2x0 to 360xc2x0.
The object of the invention is to assure the simplest possible triggering and evaluation of an angle sensor that has a nonambiguity range of 360xc2x0.
This object is attained by a method for determining the direction of an exterior magnetic field B, using a magnetoresistive sensor, having the following steps:
superimposing a fluctuating magnetic field on the exterior magnetic field B;
generating a first and second signal, dependent on the sine and cosine of the angle of the exterior magnetic field relative to a reference direction, eliminating or ignoring any signal components dependent on the fluctuating magnetic field;
ascertaining a third signal, which can be associated with the angle of the exterior magnetic field relative to the reference direction, on the basis of the first and second signal, this third signal having a periodicity of 180xc2x0;
ascertaining a correction logic signal, taking into account the signal component, eliminated from the first and second signal and dependent on the fluctuating magnetic field, which signal component assumes a first level for an angle of the exterior magnetic field B in a first angle range, in particular between 0 and 180xc2x0, and a second level in a second angle range, in particular between 180 and 360xc2x0; and
ascertaining the actual angle of the magnetic field B on the basis of the logic correction signal and of the third signal.
According to the invention, an interference-proof triggering and evaluation method for 360xc2x0 AMR angle sensors, which for instance functions independently of a temperature-dependent amplitude of the sensor signals, is made available. Fixed threshold values are not needed for performing the method. The method can be achieved economically in an electronic circuit. Measuring the effect of the current in the auxiliary coil, by means of which coil the fluctuating magnetic field is generated, is done continuously in the background, without interfering with the measurement of the uncorrected angle (that is, the angle of the magnetic field to be measured). As a result, compared with known versions that use auxiliary coils, the current in the auxiliary coil can be kept quite low, and thus thermal effects in the sensor that conventionally occur can be avoided. Also because of the low coil current, the total power loss is markedly reduced compared with conventional versions.
Advantageous features of the method and of the arrangement according to the invention are the subject of the dependent claims.
In preferred embodiment of the method of the invention, the first and the second signal, which can be assigned to the sine and the cosine, respectively, of the exterior magnetic field are additionally taken into account for ascertaining the logic correction signal K5. The first and second signal are each delivered to comparators, which by comparison of these signals with threshold values generate logic signals. By logical linkage of the logic signals thus obtained with logic signals obtained on the basis of the eliminated signal components, the correction logic signal can be furnished in a way that is computationally uncomplicated.
In a further preferred feature of the method of the invention, for ascertaining the logic correction signal, a further digital signal K6 is used, which assumes a first or second level as a function of an angle that can be associated with the fifth signal. With this provision, two comparators, which must be furnished if two logic signals on the basis of the first and second signals are furnished, can be dispensed with.
Expediently, the further logic signal assumes a first level for an angle range from 135 to 180xc2x0, and a second level for an angle range from 0 to 135xc2x0.
The object of the invention is also attained by a circuit arrangement having the characteristics of claim 5. This circuit arrangement is relatively inexpensive to furnish and at little effort and expense makes it possible to create a 360xc2x0 angle sensor, using an AMR sensor, which intrinsically has a nonambiguity range of 180xc2x0, and a corresponding logic correction signal.